A means of uniquely marking fluid particles so that both components 

 of velocity are illustrated was accomplished by inserting two cathode wires 

 (one kinked, the other straight) in the two-dimensional flow. The resultant 

 bubble pattern is a combination of time and streaJslines . (Time lines are 

 the loci of fluid particles which were located at the platinum wire at a 

 previous time (Reference k) ^) The two wires were installed so that the 

 resulting hydrogen bubbles were contained in planes which practically coin- 

 cided. Enough space should be allowed between these plans so that the 

 presence of one wire does not influence the motion of the bubbles from the 

 other wire. The direction of observation should be perpendicular to both 

 planes of bubbles. A sketch is shown in Figure 3a. 



The longitudinal displacement of the intersections of bubble lines is 

 proportional to the longitudinal component of velocity, and the transverse 

 displacement of these same intersections is proportional to the transverse 

 component of velocity. The incorporation into this analysis of the longi- 

 tudinal streaklines from the kinked wire enables determination of the 

 transverse component of velocity at the downstream extremities of the kinked 

 wire. Obviously, this particular technique for marking specific fluid 

 particles will be inadequate when the flow has velocity components of com- 

 parable magnitudes in all three spatial directions. 



The analysis to obtain the longitudinal and transverse velocity profiles 

 at a single location in steady or unsteady flow then proceeds as follows. As 

 a transverse bubble row is swept off the straight wire, intersections with 

 each of the longitudinal lines are visible looking perpendicular to both 

 planes of bubbles. These intersections are then dragged downstream, in 

 accordance with the velocity profile . When the subsequent rows of bubbles 

 are dragged off the straight wire, another series of Intersections is 

 observed. Since these intersections are fonned after an interval of time 

 equal to the pulse period, the transverse displacement referenced to some 

 visible dat-um point divided by the pulse period is proportional to the 

 transverse velocity. This velocity is a quasi-steady one taken over the 

 pulse period and, accordingly, the pulse period should be much smaller than 

 the period of any flow oscillations. The initial transverse spacing of the 



